1. Field of the Invention
The present invention relates to an ultrasonic blood flow imaging apparatus for obtaining blood flow information inside an object to be examined (living body) by using an ultrasonic Doppler effect, and displaying the blood flow information as a two-dimensional image.
2. Description of the Related Art
An ultrasonic blood flow imaging apparatus of this type is capable of performing both an ultrasonic Doppler method and a pulse reflection method. Such an apparatus can obtain both blood flow information and tomographic image (B mode image) information by using one ultrasonic probe. In addition, the apparatus can superpose color-processed blood flow information on a tomographic image and display the resultant image in real time. The principle of measurement of a blood flow velocity as one of the pieces of blood flow information in such an apparatus will be described below.
When an ultrasonic pulse (beam) is transmitted from the ultrasonic probe to a blood flow in an object to be examined, a center frequency fo in the transmission of this ultrasonic beam is dispersed by flowing blood cells. The dispersed center frequency fo is subjected to Doppler shift to change by a frequency fd. Therefore, a center frequency f in the reception of ultrasonic echoes received by the ultrasonic probe at this time is given by f=fo+fd. In this case, the center frequency in the transmission of the ultrasonic beam and the Doppler shift frequency fd are represented by the following equation: ##EQU1## where v: blood flow velocity
.theta.: angle defined by ultrasonic beam and blood vessel PA1 c: velocity of sound.
By detecting the Doppler shift frequency fd, therefore, the blood flow velocity v can be measured. The blood flow velocity v obtained in this manner is displayed as a two-dimensional image as follows. As shown in FIG. 1, ultrasonic pulses are sequentially transmitted from an ultrasonic probe 1 to an object in directions a, b, c, . . . (sector scanning). It is apparent that a scan scheme to be employed is not limited to sector scanning, but other scan schemes such as linear scanning may be employed.
When ultrasonic pulses are transmitted in the direction a a plurality of times at first, these ultrasonic pulses are Doppler-shifted by a blood flow in the object. Subsequently, ultrasonic echoes are received by the same ultrasonic probe 1. The received echoes are converted into an electrical signal, and the electrical signal is transmitted to a reception circuit 2, as shown in FIG. 2. An output from the reception circuit 2 is supplied to a phase detector circuit 3, in which a Doppler shift signal is detected. Note that in the apparatus shown in FIG. 2, the illustration of a transmission system is omitted.
The detected Doppler shift signal is sampled at each of predetermined sample points in the transmission/reception direction of ultrasonic pulses, e.g., 256 sample points SP. A Doppler shift signal sampled at each sample point is filtered by an MTI (moving target indicator) filter arranged in the phase detector circuit 3. As a result, some of the slow clutter signals are removed. Each Doppler shift signal from which some of the clutter signals are removed is frequency-analyzed by a frequency analyzer 4. Blood flow information is obtained by the analysis and is supplied to a digital scan converter (DSC) 5. The information is subjected to scan conversion in the DSC 5 and is sent to a monitor 6.
On the monitor 6, a blood flow image in the direction a is displayed in real time as a two-dimensional image. Subsequently, the same operation is repeated in the directions b, c, . . . , thus displaying blood flow images (blood flow velocity distribution images) corresponding to the respective scan directions. It should be noted that each Doppler shift signal obtained by the phase detector circuit 3 contains unnecessary noise signals, and each Doppler shift signal consists of a Doppler shift signal component (blood flow signal) based on a blood flow, and a Doppler shift signal component (clutter signal) reflected from the wall of a movable organ such as a heart. The MTI filter is an effective means for removing clutter signals.
If blood flow information contains clutter signals in addition to blood flow signals, displayed blood flow information has poor precision. Therefore, high-precision diagnosis cannot be performed. In order to minimize such clutter signals, the MTI filter is used. Another conventional means for solving such a problem is based on the fact that noise signals have small power. According to this means, blood flow information having small power itself is not displayed.
In the above-mentioned means, however, since all pieces of blood flow information having small powers are not displayed, blood flow information based on blood flow signals having small power are lost. For this reason, a portion of an image from which such blood flow information is lost is sometimes displayed in brack.
In addition, blood flow signals and clutter signals are non discriminated in the conventional art. For this reason, when, for example, the ultrasonic probe is moved, or the object breathes, or a portion near the heart is set as an imaging target, clutter signals generated due to the influences of such movement are also displayed as blood flow information. Therefore, blood flow information based on only blood flow signals cannot be properly observed. This adversely affects a diagnosis.